Key takeaways
- You must tell HMRC by 5 October 2026 if you need to file a Self Assessment return for 2025 to 2026 and have not filed one before.
- A failure to notify penalty is a percentage of the tax unpaid because of the failure, set by Schedule 41 to the Finance Act 2008.
- For a non-deliberate failure you tell HMRC about yourself within 12 months, the penalty can be 0% to 30% of the unpaid tax.
- Deliberate failures cost 20% to 70%, and deliberate and concealed failures 30% to 100%, depending on whether HMRC prompted the disclosure.
- HMRC reduces the penalty for the quality of your disclosure: up to 30% for telling, 40% for helping and 30% for giving access to records.
- GOV.UK says you may get a penalty if you register after 5 October and do not pay all of your tax bill by 31 January, so paying in full by 31 January 2027 limits the exposure.
- There is no penalty if the failure was not deliberate, you had a reasonable excuse and you told HMRC without unreasonable delay once it ended.
HMRC charges a failure to notify penalty when you do not tell it in time that you owe tax, for example by registering for Self Assessment after 5 October. The penalty is a percentage of the tax that went unpaid because of the delay: up to 30% for a non-deliberate failure and up to 100% for a deliberate and concealed one. Telling HMRC yourself, early and in full, can bring a non-deliberate penalty down to nothing.
What is a failure to notify penalty?
It is a penalty for not telling HMRC that you have a new tax liability by the deadline. The rules are in Schedule 41 to the Finance Act 2008, introduced in HMRC’s Compliance Handbook at CH71100.
For Self Assessment, the duty is to register. GOV.UK’s registration page says you must tell HMRC by 5 October 2026 if you need to file a return for 2025 to 2026 and have never filed one, or did not need to file for 2024 to 2025. Our guide on how to register for Self Assessment walks through the forms.
When do you get one?
You may get a penalty if you register after 5 October and do not pay all of your tax bill by 31 January. That is how GOV.UK’s Self Assessment penalties page puts it. The penalty is based on the amount still unpaid and is issued within 12 months of HMRC receiving your return.
So a late registration on its own does not always cost money. If you register late but pay all the tax for 2025 to 2026 by 31 January 2027, there is little or no unpaid tax for the penalty to bite on. Check whether you need to register at all with our Self Assessment thresholds guide.
How much is the penalty?
The penalty depends on your behaviour, whether HMRC prompted you and, for non-deliberate cases, how long you took. These ranges come from HMRC factsheet CC/FS11, updated 19 March 2026.
| Behaviour | Unprompted | Prompted |
|---|---|---|
| Non-deliberate, under 12 months | 0% to 30% | 10% to 30% |
| Non-deliberate, 12 months or more | 10% to 30% | 20% to 30% |
| Deliberate | 20% to 70% | 35% to 70% |
| Deliberate and concealed | 30% to 100% | 50% to 100% |
The percentage is of the “potential lost revenue”, meaning the tax unpaid because of the failure. For non-deliberate cases, the 12 months run from when the tax was due.
A disclosure is unprompted if you tell HMRC before you have reason to believe it has found out, or is about to. A letter from HMRC asking about undeclared income, such as one of its nudge letters, can make a later disclosure prompted.
How can you reduce the penalty?
HMRC reduces the penalty within its range for the quality of your disclosure. CC/FS11 splits the reduction into three parts:
- Telling: up to 30%, for admitting the failure and explaining it.
- Helping: up to 40%, for answering questions and working out the tax.
- Giving: up to 30%, for giving HMRC access to your records.
A full disclosure takes the penalty to the bottom of its range. If disclosure takes 3 years or more, the reduction is usually capped at 10 percentage points above the bottom of the range.
Example. You started a side business in 2025 to 2026, owe £4,000 and register in December 2026 after an HMRC letter. That is non-deliberate and prompted, within 12 months, so the range is 10% to 30%. With full co-operation the penalty would be 10%, or £400. Paying the £4,000 by 31 January 2027 would leave little or no unpaid tax for a penalty.
Can you avoid it with a reasonable excuse?
Yes, if three conditions are met. CC/FS11 says there is no penalty if the failure was not deliberate, you had a reasonable excuse and you told HMRC without unreasonable delay once the excuse ended. Read what HMRC accepts as a reasonable excuse for the examples GOV.UK gives.
Is the regime changing?
Possibly. HMRC’s behavioural penalties consultation, which ran from 26 March to 18 June 2025, proposed removing the 10% minimum for non-deliberate failures disclosed after 12 months. It also proposed higher penalties for deliberate cases.
At Budget 2025 the government said it intends to “simplify and strengthen” inaccuracy and failure to notify penalties. As of 6 November 2026, the consultation page shows no summary of responses, and the rates above still apply.
What to do now
- If you had new income in 2025 to 2026 and have not registered, register now. The deadline was 5 October 2026.
- Work out the tax and pay it by 31 January 2027, even if your return will be late.
- Keep a note of when and how you told HMRC. Timing decides whether the disclosure is unprompted.
- If a penalty arrives, check the behaviour and reductions HMRC has used. You usually have 30 days to ask for a review or appeal. Our guide on how to appeal an HMRC penalty explains the steps.
Frequently asked questions
What is the deadline to register for Self Assessment for 2025 to 2026?
It was 5 October 2026. If you register after that, HMRC gives you 3 months from the date of its letter to file, but the tax is still due by 31 January 2027.
Is there always a penalty for registering late?
No. GOV.UK says you may get a penalty if you register after 5 October and do not pay all of your tax by 31 January, and it is based on the amount still unpaid.
How much is the failure to notify penalty if I tell HMRC myself?
For a non-deliberate failure told to HMRC within 12 months, the range is 0% to 30% of the unpaid tax. A full disclosure can bring it to 0%.
What makes a failure to notify deliberate?
It is deliberate if you knew you had to tell HMRC and chose not to. If you also hid it, for example with false records, it is deliberate and concealed, with penalties up to 100%.
Can I appeal a failure to notify penalty?
Yes. Where a decision is appealable you usually have 30 days to send new information, ask for an HMRC review or appeal to the tax tribunal.
Sources
Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance on 6 November 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.